In the form of a swan, the king of gods was chased by an eagle, and sought refuge with a woman named Leda. The swan gained her affection, and the two mated. Leda then produced an egg, from which Helen emerged. Sometime in Helen's youth, she is abducted by a couple of Athenians. This is what begins the Trojan War.
In the comics
In DC Comics, the Silver Swan was named after Helen of Troy.